This invention relates to the art of optical fiber arrays, and to methods for making same.
It is often desirable to have very precise two-dimensional arrays of optical fibers e.g., for use with an all optical switch. In particular, for single-mode optical fiber as is typically used in optical communications networks, such fiber often having a core with a diameter of 6-9 microns and a cladding with a diameter of 125 microns, positional tolerances of less than 2 microns from true position and angular tolerances of less than 0.5 degrees are required for each fiber in the fiber arrays. In the prior art, fiber arrays were made by fabricating a plate into which holes are made, and an individual fiber end is inserted into each hole. The plates may be made from a variety of materials, with silicon or a ceramic being preferred when a very precise array is required. The holes may be made by etching or drilling into the plate, using either mechanical techniques or through the use of a laser. The individual fiber ends are locked into place, e.g., with a small amount of glue. After that, the remaining fiber stubs protruding from the front of the plate are cut off, and the resulting ends are polished flat.
Unfortunately, the plates that can be made are usually rather thin, due to limitations in the technology for the plates and their holes. Such a thin plate is able to provide only a rather short guide and hold for each fiber so that, disadvantageously, the mechanical properties of the resulting fiber array is less than desirable. Further disadvantageously, the plates have to be custom-made, which usually requires special tools and expertise. Assembly of the array also requires special skills and precise fixtures. The polishing step at the end of the assembly is not trivial, and it is very time-consuming.
Also, in the prior art, fibers have been grouped in bundles for various purposes, e.g., by tying the fibers together or by grouping the fibers inside of a sleeve, e.g., in a fiber cable. However, such groupings do not provide precise alignment and spacing of the fibers at the exit from the bundle. Also, the maximum spacing is limited to the diameter of the individual fibers.
In xe2x80x9cHigh-Density Digital Free-Space Photonic-Switching Fabrics Using Exciton Absorption Reflection-Switch (EARS) Arrays and Microbeam Optical Interconnectionsxe2x80x9d by Masayasu Yamaguchi, Tsuyoshi Yamamoto, Katsuhiko Hirabayashi, Shinji Matsuo, and Kunio Kobayu published in the IEEE Journal of Selected Topics in Quantum Electronics, Vol. 2, No. 1, April 1996, describes a 2-D fiber array consisting of stacked microglass ferrules arranged with a square packing using zirconia plates and brass frames. Disadvantageously, the fiber positional reproducibility achievable, i.e., the average displacement of the fiber centers from the desired grid points, is xc2x13.1 xcexcm, and the fiber misorientation is 4 degrees on average. Such a fiber array does not meet the stringent requirements of current MEMS-based optical switches, such as the Lambda Router from Lucent Technologies, which requires that the fiber positional reproducibility be no more than xc2x12 xcexcm and that the angular misorientation be no greater than 0.5 degrees on average.
There is a children""s project in the prior art that involves hollow cylindrical beads which may be hexagonally arranged using a form that has protruding pins, one pin for each bead. The beads are held together by first ironing the side of the beads opposite to the form, then removing the beads from the form and ironing the side of the beads that had been adjacent to the form. Such beads are not precisely spaced, or aligned and they become deformed when they are ironed. This children""s project is unrelated to optical fiber in any way.
We have recognized that, in accordance with the principles of the invention, a precise fiber array may be formed by employing a precise array of ferrules arranged with a hexagonal packing structure into ones of which is inserted and bonded, e.g., glued, a fiber end. We have further recognized that the target array, e.g., the array of micro mirrors on a corresponding MEMS device such as is employed in the Lucent Lambda Router, or other detectors or source arrays, will have to be configured to be hexagonal so as to correspond to the hexagonal fiber array. In one embodiment of the invention, a chuck is employed, at least initially, to tightly hold as an array a group of precision ferrules. Thereafter, a fiber end is inserted and bonded into ones of the ferrules. The ferrules may also be bonded to each other. If so, once the ferrules are bonded together, the chuck may be removed. Advantageously, such arrays of optical fibers may be manufactured to very high tolerances so as to be useful in positioning fiber arrays for all-optical switching. More specifically, the fiber positional reproducibility, i.e., the average displacement of the fiber centers from the desired grid points is no more than xc2x12 xcexcm and the angular misorientation is no greater than 0.5 degrees on average.
The terminating end of the fibers may be polished. Alternatively, previously cleaved terminating fiber ends may be employed, with the various terminating ends being coordinated, e.g., by an optical flat or other surface which is placed at, or adjacent to, the fiber terminating end of the ferrule array.
The ferrules employed may be conventional off-the-shelf ceramic ferrules which have low cost. Such ferrules are manufactured to very tight tolerances. More specifically, it is well established that the precision ferrules a) can be manufactured substantially uniformly, so as to have only a very small error in their diameter from the prescribed nominal ferrule diameter; b) have only a very small error, from the prescribed nominal diameter, in the diameter of the hole which runs through the ferrule and is substantially precisely in the center of the ferrule, and c) are longer than the thickness of the prior art face plates so that mechanical support superior to that achieved using such prior art faceplate arrangements is achieved. Advantageously, the precision fiber arrays of the invention scale well so that precision fiber arrays with a large number of fibers and which meet the strict Lambda Router quality requirements can be inexpensively manufactured.
In one embodiment of the invention the ferrules have one end which is at least somewhat pointed, i.e., a tip, and a conical entrance to the hole at the end opposite to the pointed end. Advantageously, such an embodiment allows for easy insertion of the fiber end via the conical entrance and the pointed tip reduces the amount of polishing of the fiber end that needs to be done.
In accordance with an aspect of the invention, the chuck may be fabricated so that it holds the ferrules in a straight orientation or so that it holds the ferrules in an angled orientation. An angled orientation provides the advantage of reducing back reflection in the fiber. It is often desirable to ensure that the fiber terminating faces of all of the ferrules are substantially coplanar.